um: track 'parent' device in a local variable

In preparation for the removal of 'driverfs_dev' from 'struct gendisk'
use a local variable to track the parented vs un-parented case in
ubd_disk_register().

Cc: Jeff Dike <jdike@addtoit.com>
Cc: Richard Weinberger <richard@nod.at>
Cc: Bart Van Assche <bart.vanassche@sandisk.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>